U.S. patent number 4,195,699 [Application Number 05/920,296] was granted by the patent office on 1980-04-01 for drilling optimization searching and control method.
This patent grant is currently assigned to United States Steel Corporation. Invention is credited to Joseph A. Fowler, Charles D. Rogers.
United States Patent |
4,195,699 |
Rogers , et al. |
April 1, 1980 |
**Please see images for:
( Certificate of Correction ) ** |
Drilling optimization searching and control method
Abstract
A searching method for optimizing the rate of penetration of a
drill into a given medium based upon the two drilling parameters of
thrust and speed of revolution. Preset base values of both
parameters are input into the drilling control mechanism at the
start of the process. Thereafter, based upon readings from an
automatic penetration rate calculator, incremental changes are
automatically made to one of the parameters, the other being held
constant, until a maximized rate of penetration is established.
Subsequently, the other parameter, previously held constant, is
changed until a new maximized penetration rate is found. This
optimized penetration rate searching is continuously and
automatically effected until the drilling process is completed. The
method additionally provides for the allowance of a wait period
between successive incremental changes for accommodating response
times of the system as well as instabilities which may occur as the
result of nonlinear process dynamics.
Inventors: |
Rogers; Charles D. (Monroeville
Borough, PA), Fowler; Joseph A. (Monroeville Borough,
PA) |
Assignee: |
United States Steel Corporation
(Pittsburgh, PA)
|
Family
ID: |
25443527 |
Appl.
No.: |
05/920,296 |
Filed: |
June 29, 1978 |
Current U.S.
Class: |
175/27; 173/177;
173/9; 700/33; 700/70; 73/152.45 |
Current CPC
Class: |
E21B
44/00 (20130101) |
Current International
Class: |
E21B
44/00 (20060101); E21B 003/06 () |
Field of
Search: |
;173/4-9,11,12
;175/24,27,26,51,114 ;73/151,151.5 ;408/8,9,10 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kennedy-Computerdrilling System, Oil and Gas Journal, May 10, 1971,
pp. 61-64..
|
Primary Examiner: Leppink; James A.
Assistant Examiner: Favreau; Richard E.
Attorney, Agent or Firm: Danchuk; William A.
Claims
We claim:
1. Method for optimizing rate of penetration of a drill within a
given medium based upon the drilling parameters of revolutions of
the drill per given time period and the thrust applied to the drill
parallel to the direction of penetration, said method
comprising:
applying a given preset start-up value for each drilling parameter
to the drill;
monitoring the rate of penetration of the drill into such medium
based upon the two parametric values being input into the
drill;
applying incremental changes in the value of one of the drilling
parameters to the drill, while keeping the other parameter value
constant, until the penetration rate of the drill into such medium
as monitored is maximized for such one parameter being
incrementally changed;
applying incremental changes in the value of the other of the
drilling parameters to the drill, while keeping the first parameter
value constant, until the penetration rate as monitored is
maximized for such second parameter being changed; and
continually alternatively applying incremental changes in the value
of one of the drilling parameters to the drill while keeping the
remaining parameter value constant until a maximized penetration
rate as monitored is realized for the parameter being changed based
upon prior changes made to the other parameter.
2. The method according to claim 1 wherein the incremental changes
effected in the value of one parameter are made independent of the
value of the other parameter.
3. The method according to claim 1 wherein the step of monitoring
the rate of penetration of the drill into the medium includes a
deadband area of insensitivity for the monitoring of the
penetration rate which precludes additional incremental changes in
the parameter values due to minor instabilities which occur as a
result of the nonlinear process dynamics of drilling.
4. The method according to claim 1 wherein the incremental changes
made to the thrust values are equal to .+-.100 psi and the
incremental changes made to the RPM parameter are equal to .+-.50
RPM.
5. The method according to claim 1 wherein the method further
includes the step of waiting a given time period between
incremental changes in the values of the two drilling parameters
before commencing yet another incremental change in order to permit
stabilization of the drilling procedure and in order to compensate
for response time of the system.
6. The method according to claim 5 wherein the period of waiting is
five seconds.
7. The method according to claim 1 wherein the step of applying
incremental changes in the value of one parameter may include
increases as well as decreases in the value of the parameter being
changed.
8. The method according to claim 7 wherein incremental changes made
in the value of one parameter are made in a direction to increase
the rate of penetration until the monitored rate of penetration
ceases to increase correspondingly, the incremental changes then
being reversed or the parameter value being held constant until a
maximum monitored penetration rate stabilizes at a relative maximum
for the two parameter values being input into the drill.
9. The method according to claim 1 wherein the method further
includes the step of periodically reading torque pressure to the
drill and automatically overriding the last parameter incremental
change when the change in torque is greater than a given preset
value for preventing damage to the drill.
10. The method according to claim 9 wherein the given preset start
up value is 300 psi/sec.
11. A method for drilling and automatically searching for an
optimized rate of penetration of a drill within a given medium,
based upon a first drilling parameters of drill speed and a second
drilling parameter of thrust, for maximizing the efficiency of
energy transfer from the drilling equipment to the drill/medium
interface, said method comprising:
progressing the drill into the medium until the drill is
collared;
applying a given preset start-up value for each drilling parameter
to the drill;
monitoring the rate of penetration of the drill into such medium
based upon the two parametric values being input into the
drill;
applying automatic incremental changes in the value of the first of
the drilling parameters to the drill while keeping the second
parameter value constant until the penetration rate is maximized
for such one parameter being incrementally changed;
applying automatic incremental changes in the value of the second
drilling parameter to the drill, while keeping the first parameter
value constant, until the penetration rate is maximized for such
second parameter being changed; and
continually alternatively applying automatic incremental changes in
the value of one of the drilling parameters to the drill while
keeping the remaining parameter value constant until a maximized
penetration rate is realized for the parameter being changed based
upon prior changes made to the other parameter.
12. The method according to claim 11 wherein the incremental
changes efforted in the value of one parameter are made independent
of the value of the other parameter.
13. The method according to claim 11 wherein the step of
calculating monitoring the rate of penetration of the drill into
the medium includes providing for a deadband area of insensitivity
for the monitoring of the penetration rate which precludes
additional incremental changes in the parameter values due to minor
instabilities which occur as a result of the nonlinear process
dynamics of drilling.
14. The method according to claim 11 wherein the incremental
changes made to the thrust values are equal to .+-.100 psi and the
incremental changes made to the RPM parameter are equal to .+-.50
RPM.
15. The method according to claim 11 wherein the progression of the
drill into the medium until the drill is collared is achieved
automatically.
16. The method according to claim 11 wherein the progression of the
drill into the medium until the drill is collared is achieved
manually.
17. The method according to claim 11 wherein the method further
includes the step of automatically waiting a given time period
between incremental changes in the values of the two drilling
parameters before commencing yet another automatic incremental
change in order to permit stabilization of the drilling procedure
and in order to compensate for the response time of the system.
18. The method according to claim 17 wherein the period of waiting
is five seconds.
19. The method according to claim 11 wherein the step of applying
incremental changes in the value of one parameter may include
increases as well as decreases in the value of the parameter being
changed.
20. The method according to claim 19 wherein incremental changes
made in the value of one parameter are made in a direction to
increase the rate of penetration until the monitored rate of
penetration ceases to increase correspondingly, the incremental
changes then being reversed or the parameter value being held
constant until the maximum monitored penetration rate stabilizes at
a relative maximum for the two parameter values being input into
the drill.
21. The method according to claim 11 wherein the method further
includes the step of periodically reading torque pressure to the
drill and automatically overriding the last parameter incremental
change when the change in torque is greater than a given preset
value for preventing damage to the drill.
22. The method according to claim 21 wherein the given preset value
start up is 300 psi/sec.
Description
BACKGROUND OF THE INVENTION
A recent renewed interest in coal mining has once again brought out
the salient limitations in current procedures for mining coal.
Specifically, the most nonproductive segment of time within the
coal mining process is associated with roof bolting. The time
consumed during this necessary process far exceeds, in proportion,
any other time segment dedicated to the other details incorporated
within the mining of coal. This delay in the advance of underground
continuous mining equipment represents a reduction in mine
productivity. Moreover, the instability of the drilling process
itself is reflected in the myriad of problems associated with the
drilling of roof bolt holes. Specifically, the melting of solder
holding the carbide cutting tip on the drill bit, the bending of
the drill steel as well as the total lodging of the drill steel
within the hole have all resulted in wasted time as well as
expense.
The major delay in roof bolting stems from the time consumed in
drilling holes for roof bolts. With the present roof drilling
machines, the operator utilizes a "feel" of vibration and visual
estimates of roof drill penetration rates to adjust the thrust and
torque input into the drilling system. While such a nonscientific
system produces "adequate" results, the amount of delay in the time
consumed in roof hole drilling suggests otherwise. This delay is
especially important in other "long" drilling applications. In an
ideal situation, the advance of the roof drill into the rock or
medium being drilled should be maintained at a maximum penetration
rate at all times. It should be obvious from past performances that
this optimization has not been obtained.
The specific causes which have resulted in the time consumption
inherent in roof bolt drilling comes from an assumed, but
incorrect, presumption that roof bit hole drilling is an art and
not a science. This is substantiated in part by the present-day
thought that the best roof bolt hole driller is the one man who has
the best "feel" for the progress and vibration of the drill and
drill steel into the rock. It has recently been determined that the
maximized penetration rate of a drill and associated drill steel
into any medium being drilled, whether that medium be limestone,
sandstone or coal itself, is susceptible to a scientific and
logical set of parameters and algorithms which do in fact result in
an optimized rate of penetration into the medium being drilled.
While a variety of prior art is available in this area, none fully
appreciate the use of drilling parameters and the values of the
drilling parameters which may be manipulated in a specific manner
in order to accomplish a maximized penetration rate of the drill
into the medium.
SUMMARY OF THE INVENTION
The present invention is addressed to a method for optimizing the
rate of penetration of a drill within a medium to be drilled. The
method according to the present invention utilizes an apparatus
which automatically provides the roof-drill operator with a
numerical display of penetration rate during the drilling process
and automatically conducts a self-adjustment of the roof-drilling
controls to achieve such maximized penetration rate of the drill
bit into the medium.
The basis of the present method resides in an anomally in that the
two drilling parameters, i.e., drill thrust and drill speed (RPM),
are independently operative variables but have an interdepending
relationship with one another. Specifically, the present method
utilizes a searching program for the optimized penetration rate
which encompasses the incremental change of one of the variables
noted above while retaining the other variable in a constant state.
When the penetration rate has been maximized for the one variable
being changed, the process of parameter variability is then applied
to the previously unchanged variable while the previously changed
variable is held constant. For instance, the RPM of the drill may
be incrementally changed in both a positive and negative sense
while the thrust is held constant. The RPM is incrementally varied
until the penetration rate, as indicated by the calculation
apparatus, is stabilized at a maximum reading. Once this stage has
been reached the RPM value being input to the drilling apparatus is
then held constant while the thrust applied to the drilling
apparatus is varied incrementally until a new maximized penetration
rate is realized. This process is repeated during the seaching
cycle of drilling.
The method of the present invention also incorporates a waiting
period between incremental changes in the values of the two
drilling parameters before commencing yet another incremental
change. The waiting period is utilized to compensate for any
changes in the time response of the process being controlled. This
is specifically applicable with respect to changes in the response
time of the hydraulic system valves. Additionally, changes in the
thermal characteristics of the drill/rock interface, i.e., the rate
of heat removal, may also affect the response time of the system.
The method according to the present invention also includes
creating a dead-band area of insensitivity for the calculation of
the penetration rate for permitting stabilization of the drilling
procedure to preclude the inclusion, within the calculation of the
penetration rate, of instabilities which occur as a result of
nonlinear process dynamics. These may occur due to small variances
in hardness, etc., of the rock or coal medium being drilled which
may change the proportionality between thrust and penetration rate
or between RPM and penetration rate.
The method according to the present invention utilizes a
microcomputer for calculation of the penetration rate as well as
the timing sequence of the incremental changes to the two drilling
parameters. This very precise control over the drilling parameters
obviates the need for inaccurate and mistaken estimates by the
drill operator himself as to penetration rate and the "feel" of
vibration and visual estimates of roof drill penetration rate. Due
to the precise control over the drilling parameters and the input
to the drilling control system, there is realized a much improved
speed of penetration of the drill bit into the medium.
Accordingly, it is a general object and feature of the present
invention to provide a method for optimizing the rate of
penetration of a drill within a given medium by selectively and
incrementally changing the drilling parameters of drill speed and
thrust by continually alternatively applying incremental changes in
the values of the drilling parameters independent of each other
until a maximized penetration rate is realized for each of the
parameters with respect to the value of the remaining parameter
being held constant.
It is another general object and feature of the present invention
to provide a method for drilling and automatically searching for an
optimized rate of penetration of a drill within a given medium,
based upon the drilling parameters of drill speed and thrust for
maximizing the efficiency of energy transfer from the drilling
equipment to the drill/medium interface.
It is yet another object and feature of the present invention to
provide a method for drilling based upon the two drilling
parameters of drill speed and drill thrust which utilizes specific
incremental changes to the parameters and which also includes a
given wait time between the incremental changes in the values of
the two parameters before commencing yet another incremental change
in order to permit stabilization of the drilling procedure and to
preclude inclusion within the calculation of the penetration rate
of instabilities which occur as a result of non-linear process
dynamics and to compensate for any changes in time response.
Other objects and features of the present invention will, in part,
be obvious and will, in part, become apparent as the following
description proceeds. The features of novelty which characterize
the invention will be pointed out with particularity in the claims
annexed to an forming part of the specification.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features that are considered characteristic of the
invention are set forth with particularity in the appended claims.
The invention itself, however, both as to its structure and its
operation together with the additional objects and advantages
thereof will best be understood from the following description of
the preferred embodiment of the present invention when read in
conjunction with the accompanying drawings wherein:
FIG. 1 is a functional block diagram of a microcomputer-based drill
control system which utilizes the method according to the present
invention;
FIG. 2 is a flow diagram of a simplified optimal gradient method
according to the present invention;
FIG. 3 is a further flow diagram indicating the parameters
incorporated within the present invention;
FIGS. 4a, 4b, and 4c are assumed transfer relationships between the
manipulated and control variables of the drilling process which
form the basis of the present invention; and
FIG. 5 is a series of charts indicating an empirically-derived
interrelationship between the hydraulic pressure channelled to the
drill motor, the drill thrust, the rate of penetration and the
speed of the drill.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, there is shown a functional block diagram of a
microcomputer-based drill control 10 which utilizes and is operated
according to the method of the present invention. The
microcomputer-based drilling control is employed to effect the
input to a drill motor 12 and a thrust cylinder 14. The drill motor
12 is used to rotate the drill bit and drill steel while the thrust
cylinder 14 provides the necessary force to the drill bit in the
direction of drilling.
The drill head 12 is fitted with electrically operated hydraulic
valves which, in turn, are operated by signals from an operator's
control panel. Two such hydraulic valves are presented at 16 and
18. Valve 16 is a pressure compensated electrohydraulic flow valve
while valve 18 is an electrohydraulic pressure reducing valve. The
signals generated from the operator`s control panel to the two
hydraulic valves 16 and 18 are augmented with electrical voltages
from a microcomputer 20. In a preferred embodiment of the present
invention, the microcomputer 20 is a Motorola Model 6802
microprocessor. Through means of a special control algorithm stored
within the microcomputer memory, the output voltages from the
computer via electrical line 22 are used to manipulate the drill
rotational velocity (RPM) and vertical thrust (B). The control
algorithm is employed to calculate the RPM and thrust so as to
maintain a maximum rate of penetration (fee per minute) as the
drill is operated in various rock types.
The objective of the present method effected through use of the
microcomputer 20 is to develop a practical system which is capable
of providing optimum control of the roof drilling operation. In
this case, optimum control implies the fastest or optimum drilling
rate in terms of the constraints imposed by the rock type, bit and
rod conditions, chip size, and available torque and thrust. Such
controls, when implemented, will provide safer and a more efficient
drilling operation. Furthermore, a more uniform action of the
control compared with manual operation will reduce bit and rod
abuse. These advantages will be reflected as direct reductions in
delay at the coal face and therefore represent advantageous
increases in productivity.
Positioned on the operator panel (not shown) are two manually
reversing drill valves 24 and 26. The drill valve 24 effects
rotation of the drill bit while valve 26 is a hydraulic control
over the vertical motion, i.e., thrust applied to the drill bit.
Electrically driven operator controls as at 28 and 30 permit manual
control by the operator of the RPM and thrust during the drilling
sequence. The manually operated reversing valves as well as the
operator controls are employed for permitting the manual
positioning (collaring) and withdrawal of the drill while the
microcomputer controls the actual drill advance after collaring has
been achieved. The two electrohydraulic valves 16 and 18 which are
in series with the manual reversing valves are used to provide
precise control of the drilling rotational speed (RPM) and thrust
(B). These valves are set from an operator's panel to regulate the
hydraulic oil flow to the drill motor and oil pressure to the drill
thrust cylinder respectively. The operator RPM control 28 and the
operator thrust control 30 are electrically connected to two valve
amplifiers 32 and 34 respectively. These valve amplifiers are
necessary inasmuch as the voltages being output from the operator
controls are insufficient in and of themselves to effectively drive
the pressure compensated electro-hydraulic flow valve 16 and the
electrohydraulic pressure reducing valve 18.
Positioned in both an input and output relationship with the
microcomputer 20 is an input/output interface 36. The interface 36
receives the output from the microcomputer 20 through the
previously mentioned line 22 while the microcomputer 20 receives
its input from the interface 36 via an electrical line 38. The
input/output interface 36 functions as a digital logic to analog
logic converter through line 22 while at the same time acts as an
analog to digital converter via line 38. Consequently, the inputs
received by the interface 36 (which will be described shortly) are
in analog form and must be converted to digital form before being
passed to the digital microcomputer 20 via electrical line 38. In
much the same manner, the output of the interface (which again will
be described in further detail below) must be in analog form. The
digital output from the microcomputer 20 must be converted to
analog form by the interface 36 prior to its passage to the valve
amplifiers 32 and 34. This is not the case with respect to a visual
display unit indicated generally at 40. The output from the
input/output interface 36 are connected to the valve amplifier 32
via an electrical line 42. The output of the interface 36 is
connected to the valve amplifier 34 via a second electrical line 44
and a third output of the interface to the digital display unit 40
is made via an electrical line 46. As indicated in FIG. 1, toggle
switches 48 and 50 are provided between the line 42 and the valve
amplifier 32 and between the line 44 and valve amplifier 34,
respectively. The switches 48 and 50 are provided to permit a
solely manually operable drilling operation or any portion thereof.
The computer output along line 42, labeled V.sub.s, is indicative
of an analog voltage along electrical line 42 to the summing
junction of the electrohydraulic valve amplifier 32. Similarly, the
output voltage indicated as V.sub.B along line 44 to the
electrohydraulic valve amplifier 34 is an analog voltage output
from the interface 36 to the summing junction of the amplifier 34.
A digital voltage V.sub.D applied along line 46 to the display unit
40 is used to illuminate the rate of penetration (FPM) display.
As indicated in FIG. 1, the inputs to the input/output interface 36
come from two sources. The first source of input to the interface
36 is derived from a pressure transducer 52 positioned between the
electrohydraulic flow valve 16 and the drill motor 12. The pressure
transducer 52 measures the hydraulic pressure along the hydraulic
line 54 running between the flow valve 16 and the drill motor 12
and provides a voltage signal V.sub.t along line 56 to the
interface 36. The second input signal to the interface 36 is
derived from a string potentiometer 58 which measures the movement
of the drill bit and drill steel into the medium being drilled. The
string potentiometer 58 provides a voltage signal V.sub.L along an
electrical line 60 to the input/output interface 36. Both of the
voltages V.sub.L and V.sub.t are analog voltages which are
converted to a digital value by the interface for use in the
computer control calculations.
As alluded to previously, the values of the drilling parameters of
thrust and RPM being input into the drill motor and thrust cylinder
are hydraulically actuated along the lines 54 and 62. The specific
manner in which the hydraulics are used to control the drilling
parameters is derived from the electronics of the apparatus shown
in FIG. 1.
The high speed capability of the microcomputer 20 provides a
calculation power to repetitively conduct many control functions
within a few hundredths of a second. In a preferred emboiment of
the present invention, the microcomputer 20 executes approximately
one thousand operations in 0.03 seconds. The drill logic and
control functions which are repetitively executed within this time
period include (1) a status and reasonability check of all operator
switch positions; (2) a precise monitoring of the drill
displacement and a computation of the feed per minute which is
indicated in the display unit 40; (3) calculations that apply a
classical optimal gradient algorithm for determining the correct
combinations of drill, RPM and thrust to maximize the penetration
rate; (4) transmission of the proper combination of electrical
signals to operate the hydraulic valves and to display the current
value of the penetration rate indicated on the display unit; (5) a
monitoring of the current drill displacement and a comparison of
such displacement with the preprogrammed limit of travel; and (6) a
monitoring of the drill motor hydraulic pressure. A detailed
explanation of the optimal gradient strategy incorporated within
the method of the present invention will be discussed below.
Looking to FIG. 2, there is shown a flow diagram of a simplified
optimal gradient method according to the present invention. In
operation, once the roof drill is advanced into the rock by a few
inches (a state referred to in the art as "collared"), the
optimizing control searching program is activated as indicated by
the start command 80 in FIG. 2. The program is set to effect
alternative increments either to the amount of drill thrust or
rotational speed by a programmed amount. In a preferred embodiment
of the invention, these values are set at .+-.100 psi and .+-.50
RPM, respectively. At the start of the drilling procedure, just
after the drill is collared, preset values for both thrust and RPM
are employed in the drilling program. Subsequently, one of the
drilling parameters is manipulated as is indicated by box 64 in
FIG. 2. Once the variable or parameter has been manipulated one
increment, a brief wait period or delay indicated by box 66 is
effected. The feet per minute advance of the drill into the medium
being drilled is calculated precisely and compared with the
previously calculated value. This calculation is represented by
boxes 68 and 70 in FIG. 2. If the results of this comparison
indicate that the last variable manipulated (either the thrust or
drill speed) has provided a measurable improvement in the
penetration rate, that variable is again incremented. Otherwise,
depending upon the magnitude of the measurement, that variable is
decremented by one-half, kept the same or the opposite variable is
incremented. The analyzation of the result of the adjustment of one
incremental change to one of the parameters is indicated by box 70
in FIG. 2. Should a change in the penetration rate (.DELTA.FPM) be
at least equal to a constant K.sub.1, a signal is provided to the
adjusting box 64 via a line 72 to again increase the incremental
change. Should however the .DELTA.FPM be less than or equal to a
constant K.sub.2, there is a command given to the adjustment box
via a command indicated at 74 and line 76 to adjust the previously
manipulated variable in accordance with the change in the
penetration rate. As indicated at 78, once the varied parameter has
resulted in a maximization of the penetration rate, a setup system
for adjustment of the opposite manipulated variable or parameter is
provided for making appropriate incremental changes (along with
appropriate delays) to the adjustment command indicated by box 64.
This second parameter is incrementally varied in a similar manner
as the first parameter was varied until an optimized penetration
rate is realized. This process is continually repeated during the
drilling of the medium. First one parameter is incrementally varied
while the other is kept constant and then the situation is reversed
so that the once static variable is changed while the previously
varied parameter is held static or constant.
The choice of an algorithm for the control of the roof drill is a
result of the hypothesis that the describing function for rock
drilling is a three-dimensional relationship having a single
optimum. The control variable efficiency of the drilling operation
as inferentially described by the rate of drill penetration (FPM),
achieves a maximum steady state value and is a function of the two
manipulated variables of drill rotational velocity (RPM) and thrust
(B). The long-term variable which form a part of the drill
operating conditions and average rock characteristics remain
relatively constant and therefore are assumed at a steady state
during a one-hole drilling. Short-term or insignificant changes in
drilling conditions are regarded as process noise. Significant
increases in rock hardness are treated during the program as
temporary and a control override is implemented within the present
method.
The drilling control method of the present invention employs an
evolutionary optimization theory which is particularly amenable to
steady-state optimization of a poorly defined process such as rock
drilling. A number of theoretical strategies are available within
the evolutionary optimization method, the choice of which is
dependent on such factors as inherent process stability, noise,
computer capacity, and the type of constraints imposed upon the
system. The current strategy chosen under the evolutionary
optimization theory for the drilling process is a discretized
(incremental stepping version) of the optimal gradient strategy. As
currently implemented within the present invention, this algorithm
is relatively simple and slow acting. As will be discussed later,
because of stability considerations brought about by nonlinear
process dynamics, slow action has proven an advantage to the system
as programmed. The simple form of the algorithm is also maintained
to facilitate programming into a microcomputer memory and yet has
the advantage of consuming only a minimal amount of program
memory.
As previously noted, the necessity for the algorithm is predicated
on the existance of a three-dimensional relationship between
efficiency or rate of drill penetration (FPM) and the independent
variables or parameters of drill speed (RPM) and thrust (B). Taken
independently, the drill speed and thrust parameters were assumed
to be related to the rate of penetration by relationships similar
to those shown in FIGS. 4a and 4b. In FIG. 4a, the efficiency of
energy transferal from the drilling equipment to the actual rock
penetration (as described by the rate of the drill penetration
(FPM)) increases with thrust (B) at constant values of RPM.sub.n.
Once the drill is fully collared, the maximum penetration rate
occurs wherein further increases in thrust cause the rate of
penetration to remain constant or to decrease. A few conceivable
factors contributing to this reduction in efficiency include:
1. The drill rod begins to bend within the hole and therefore
energy is consumed in overcoming friction on the hole side.
2. With higher normal forces on the drill bit, the cutting edges
are degraded to the extent that cutting effectiveness is
reduced.
3. Principally in dry drilling, the nonoptimum cutting method
produces chip sizes which result in a situation in which removal
methods become ineffective.
Looking to FIG. 4b, a similar relationship is assumed between the
rate of penetration and RPM at constant values of thrust (B.sub.n).
The factors which contribute to the existence of a maximum on this
family of curves may include:
1. The energy level is producing heat at a faster rate than it may
be removed. Thus the bit cutting effectiveness is reduced by
operating the bit at too high a temperature; and
2. Increases in a bit rate of rotation produces a non-optimized
chip volume of a given chip size.
Again, an increased amount of energy is consumed as resultant
friction between the drill rod itself and the chips existent within
the hole sides.
By combining the two functions relating RPM and thrust to the rate
of penetration, a three-dimensional relationship has been developed
for application to the evolutionary optimization theory.
Applicants' experimental confirmation of the relationships of
penetration rate to drill speed and thrust (FIGS. 4a and 4b,
respectively) were successful. Data was collected during periods of
steady-state running of the drill as the drill progressed into the
medium. The penetration rate was recorded for specific drill speeds
and thrust settings. The results from such data collection resulted
in a conclusion that the steady-state data does not reflect a
requirement in which the RPM and thrust must be manipulated along
an optimum acceleration trajectory in order to maintain drill
stability between each steady state. Rather, it was determined
during the experiments that the RPM and thrust must be increased in
a manner consistent with the changing process constraints and
thereby avoid possible process instability. During some moves for
data gathering, process instability was evidenced by overheating of
the drill bit, bending of the drill rod or sticking of the drill
rod in the rock. The steady-state conditions are evidenced by the
flow diagram in FIG. 3.
The practical value of applicants' attempts to substantiate the
graphs noted in FIGS. 4a and 4b by the generation of steady-state
data was the development of the following set of five control
algorithms specifications. In essence, the computer control
algorithm is required to (1) act independently of highly nonlinear
process dynamics; (2) conduct independent manipulations of each
variable (RPM or B) after that particular variable has been
determined to provide the most improvement in drilling efficiency
(feet per minute) at each state of the process. In effect, this
action of the control determines the optimum trajectory for drill
acceleration in real time; (3) arrive at and maintain a final
maximum rate of drill penetration (feet per minute) once the drill
rod is fully collared; (4) protect against substantial changes in
the describing function due to effects of highly stratified rock
characteristics; (5) implement a means to prevent computer control
action as a result of measurement noise or insignificant changes in
rock characteristics.
The computer control algorithm requirement of action independent of
the highly nonlinear process dynamics is achieved by providing for
a wait period or computer control delay which is executed between
each manipulation of the thrust or drill speed. Thus, the drill
penetration rate achieves a new steady-state value before an
analysis of the effect of each manipulation is conducted.
Adjustment of the delay length or wait period is dependent upon
several factors including electrohydraulic system response, thermal
time constant of the drill bit, and collaring depth of the hole. In
a preferred embodiment of the present invention, this wait period
is in the area of five seconds. It is this wait period which
precludes instabilities which may occur as the result of nonlinear
process dynamics.
The computer control algorithm requirement of the conduction of
independent manipulations of each variable for manipulation along
the optimum trajectory to the final maximum rate of penetration is
achieved as follows: Once preset values of thrust and RPM are
reached, the thrust or drill speed is incremented by fixed amounts.
This action is continued until a negligible amount of improvements
occurs in the penetration rate. Subsequently, this same process is
repeated with the variable or parameter which had been previously
retained in a steady or static condition. The system continues to
alternate back and forth between the manipulated variables until
the final hole depth is reached. If during this iterative process
the rate of penetration decreases as a result of an increment in
RPM or thrust, either a full or one-half decrement occurs in that
particular variable dependent upon the magnitude of the rate of
penetration decrease. Once it has been determined that the rate of
penetration is at an absolute steady-state maximum, the control for
the drill continues to search for an optimum in the event that a
significant change occurs in rock characteristics and/or drill
operating conditions.
The computer control algorithm requirement for protection against
substantial changes in the describing function due to highly
stratified rock characteristics is achieved through a unique
override system. Abrupt increases in hardness of the rock which
occur during computer operation would be reflected (unless
contained) as a hazardous condition of torque on the drill steel.
The override provided within the computer control algorithm
differentiates the hydraulic pressure in the drill motor line with
respect to time and cancels or withdraws the current or last
increase in drill thrust (B). This action occurs at the instant the
differential exceeds a preprogrammed constant. In a preferred
embodiment of the present invention, this override of torque
limiting is set at changes in pressure greater than 300 psi per
second of the hydraulic pressure in the drill motor line. It must
be noted under such circumstances, however, that the computer
control continues to search for the maximum penetration rate
identified by the new describing function. As indicated in the
functional block diagram of FIG. 1, the hydraulic pressure in the
drill motor line 54 is used as an input variable to the
microcomputer system via the pressure transducer 52 and its voltage
V.sub.t. P.sub.t is time sampled by the computer at a period
considerably shorter than the maximum system time constant or wait
period. For instance, in a preferred embodiment of the present
invention, P.sub.t is time sampled by the computer at one second
intervals. If the differential .DELTA. p.sub.t /.DELTA. time
exceeds the predetermined limit of 300 psi per second, either the
current or previous increment of drill thrust is negated.
The computer control algorithm requirement of avoidance of control
action occurring as a result of electrical measurement noise,
insignificant changes in rock characteristics or insignificant
changes in drilling conditions is met by the provision of a
deadband within the control actions which prevents computer control
action of the thrust and RPM for changes in the penetration rate of
the drill which are less than or equal to one-half foot per minute.
This deadband range is important in that it obviates the need for
computer control action during such insignificant changes in rock
characteristics or in drilling conditions as a whole.
The microcomputer-based control implements the overall control
algorithm with the increment or decrement size, length of wait
period, deadband, and constant for torque override set at values to
be satisfactory for experimental running. These values have been
previously noted for the preferred embodiment of the present
invention but may be varied according to the requirements set
during any set of drilling conditions. Modifications of these
constants or control logics may easily be implemented by simple
changes within the microcomputer programmable memory.
Looking to FIG. 5, there is shown a series of curves based upon
empirically derived measurements of the variables noted therein. It
is important to note with regard to FIG. 5 that each particular
recording or channel is identified with the variable being recorded
on a relative scale. The horizontal coordinates relate to time with
time increasing in the direction of the indicated arrow. In order
to place the variables noted in FIG. 5 in proper perspective, it
should be noted that P.sub.t is the hydraulic pressure proportional
to torque input into the drill motor, P.sub.B is the hydraulic
pressure proportional to thrust, the drill displacement L is the
amount of vertical displacement of the drill or .DELTA. L over
.DELTA. t which is equal to the rate of penetration and RPM is the
rotational velocity of the motor. The specific effect upon the
drill displacement at given points will be indicated by
numerals.
Even though the hardness of the rock being drilled had major
changes throughout the drilling operation, an optimum penetration
rate was maintained. Additionally, through all its severe
excursions in the torque, the drill penetration rate was maintained
stable by the computer control and the drill bit itself remained in
good condition after drilling of several holes. Overall, the
penetration rate for the tests performed averaged between three
feet per minute and six feet per minute. Once the drill had been
collared, the computer control was initiated at point 1. At this
instant, the initial values for thrust and motor speed had
stabilized. At point 2, the initial incremental increase in thrust
(P.sub.B) of a 150 psi occurred. After a five second wait period a
second increase was called for. However, the first rise in P.sub.t
(see arrow on chart) negated the increase. A second "spike" in
P.sub.t decreased the thrust to its nearly original value at point
3. At that time, the computer control was about to increase the
RPM. At point 4, the increase occurred but after a five second
analysis of penetration rate the RPM increase was withdrawn at
point 5. Between points five and six, an increase in thrust was
prevented by the large excursion in P.sub.t (greater than 300 psi
per second). In other words, the override provision of the present
system became effective. At points 6 and 7, increases in RPM caused
a computed increase in the penetration rate. Between points 7 and
8, an additional attempt to increase thrust was negated. This again
was caused by a spike in P.sub.t and the override provision came
into operation. Points 8 and 9 represent increases in the RPM and
point 9 takes the RPM above its high limit which is also beyond the
optimum penetration rate as well. Approximately five seconds after
point 9, the RPM decrease takes the RPM just below the upper limit
(note the arrow on the RPM scale). At point 10, an increasing
thrust is attempted and at point 11, the increase is removed
because of the decrease in penetration rate that occurred as a
result of the thrust increase. At point 11, the RPM is driven again
to its upper limit. Continued increases in thrust occurred after
point 12 until the top of the rock was reached.
The graphs indicated in FIG. 6 evidence an independent nature to
the RPM and thrust incremental changes in the searching attempt of
the present method for the optimized penetration rate. In addition
to an optimization of the penetration rate, there was realized a
concomitant prevention of drill bit deterioration as well as the
prevention of bending of the drill steel or drill rod. Therefore,
the present method not only provides for an optimized penetration
rate but also prevents increases in downtime due to undesirable
breakdowns in either the drill bit itself or the drill steel or
drill rod.
In conclusion, it should be realized that the present method
describes a process for control of the roof drilling in an optimum
manner while displaying the results of the drilling operation to
the drill operator. Drill bit deterioration as well as drill steel
bending is prevented. More importantly, however, the weak link in
any drilling operation, i.e., the time required for the penetration
of a drill into a medium, is greatly shortened due to a realization
of an optimized penetration rate for the drill. The present control
method provides for a totally automated high productivity drilling
system. The present method may also provide for remote operation of
the drills which would greatly reduce the hazards of roof falls to
the drill operator. The holdup time reductions resulting directly
in increases in productivity should be obvious. However, the
indirect advantages realized from the present drilling method save
additional expenses related to the abuse normally exacted upon the
drilling operation under previously manually controlled
systems.
While certain changes may be made in the above-noted apparatus
without departing from the scope of the invention herein involved,
it is intended that all matter contained in the above description
or shown in the accompanying drawings, shall be interpreted as
illustrative and not in a limiting sense.
* * * * *